High density data storage upon magnetic media is traditionally plagued with two problems. The first is in generating the high frequency waveforms required to lock a detector to the input data. The second is the phenomena of "pulse crowding" where transitions written close together upon a magnetic media tend to drift apart. More expensive circuits and components have been used to combat the first problem. Novel data encoding schemes have been the main weapon used against the second. The prior art utilizes a method utilizing a sawtooth-lock waveform and a Miller encoding scheme. This encoding scheme places a transition in the middle of a "bit cell" when a "1" is encoded, and places a transition at the leading edge of a "bit cell" if a "0" is to be encoded and the previous "bit cell" was not a "1." This is further described in U.S. Pat. No. 3,108,261 entitled RECORDING AND/OR REPRODUCING SYSTEM issued to Arthur Miller Oct. 22, 1963. FIG. 1 illustrated an improved (modified Miller or M.sup.2) encoding method of a typical bit pattern where a transition is placed at the leading edge of a "bit cell" if a "0" is to be encoded and the previous bit contains no transitions. As can be seen, M.sup.2 encoding results in a minimal number of transitions for a given signal while still providing sufficient transitions for detector lock regardless of the data content. It should also be appreciated that the transitions indicating "1's" are subject to "pulse crowding" more than the transitions indicating "0's." The prior art has taken advantage of this characteristic in similar self-clocking codes by designing the detector-lock waveform with larger "windows" for "1" transitions than for the "0" transitions. A typical phase-lock waveform, that is used by Shugart Associates, is illustrated in FIG. 2. The quality of the encoded data is verified by encoding a bit pattern onto the magnetic media, decoding the data and comparing the original bit pattern to the bit pattern subsequently read. The disadvantages of the prior art are twofold. First, the detector-lock waveform requires the generation of extremely high frequencies and is not readily adaptable to further increases in data rates. Secondly, the prior art method of verifying data transfer operation does not detect pulse crowding quantitatively or indicate the vulnerability of certain data to being erroneously decoded in further operations.